The present invention relates to an objective lens, coupling lens, light converging optical system, and optical pick-up apparatus for the recording and/or reproducing of an optical information recording medium.
Recently, according to the practical use of the short wavelength red semiconductor laser, a DVD (digital versatile disk) whose dimension is almost the same as a CD (compact disk) which is the conventional optical disk (optical information recording medium), and whose capacity is greater and whose density is larger, is developed and comes into products, and it is presumed that, in the near future, the next generation optical disk with the higher density comes into the market. In the optical system of the optical information recording and reproducing apparatus in which such the next generation optical disk is the medium, in order to attain the densification of the recording signal or to reproduce the high density recording signal, it is required that the diameter of the spot converged onto the information recording surface through the objective lens is decreased. In order to attain this, the reduction of the wavelength of the laser as the light source, or the increase of the numerical aperture of the objective lens, is necessary. A laser whose application as the short wavelength laser light source is expected, is a blue violet semiconductor laser having the oscillation wavelength of about 400 nm.
In this connection, when the reduction of the wavelength of the laser light source or the increase of the numerical aperture of the lens is intended, it is presumed that even a problem which can be almost negligible in the optical pick-up apparatus composed of the combination of the comparatively longer wavelength laser light source and low numerical aperture objective lens, in which the information is recorded or reproduced for the conventional optical disk such as the CD or DVD, becomes more prominent.
One of them is a problem of the axial chromatic aberration generated on the objective lens due to the minute variation of the oscillation wavelength of the laser light source. A change of refractive index due to minute wavelength variation of the common optical lens material becomes large as the short wavelength is processed. Accordingly, the defocus amount of the focal point generated due to the minute wavelength variation becomes large. However, as can be seen from a fact that the focal depth of the objective lens is expressed by k·λ/NA2 (k: proportional constant, λ is the wavelength, and NA is the image side numerical aperture of the objective lens), the shorter the using wavelength is, the focal depth is smaller, thereby, the slight defocus is not allowable. Accordingly, in the optical system using the short wavelength light source such as the blue violet semiconductor laser, and the objective lens of the high numerical aperture, in order to prevent the mode hop phenomenon or the variation of the wavelength due to the output change, and the deterioration of the wave front aberration by the high frequency superimposition, the correction of the axial chromatic aberration becomes important.
Further, another problem which becomes prominent in the reduction of the wavelength of the laser light source and the increase of the numerical aperture of the objective lens is the variation of the spherical aberration of the optical system due to the temperature•humidity change. That is, a plastic lens commonly used in the optical pick-up apparatus is easily deformed due to the temperature or humidity change, and further, the refractive index is largely changed. Also the variation of the spherical aberration due to the change of the refractive index which is not so much problem in the optical system used in the conventional optical pick-up apparatus, becomes not negligible amount, in the reduction of the wavelength of the laser light source and the increase of the numerical aperture of the objective lens.
Further, yet another problem which becomes prominent in the reduction of the wavelength of the laser light source and the increase of the numerical aperture of the objective lens is the variation of the spherical aberration of the optical system due to the thickness error of the protective layer (called also [transparent substrate]) of the optical disk. It is well known that the spherical aberration generated due to the thickness error of the protective layer is generated in proportion to 4-th power of the numerical aperture of the objective lens. Accordingly, as the numerical aperture of the objective lens is increased, the influence of the error of the thickness of the protective layer is increased, and there is a possibility that the stable recording or reproducing of the information can not be conducted.
Further, because the plastic lens has larger change of the refractive index or shape due to the temperature change as compared to the glass lens, it is easily happened that the deterioration of the performance thereby becomes a problem. Because this deterioration of the performance, that is, the increase of the spherical aberration is larger as the NA is increased, (generally, it is increased in proportion to the 4-th power of NA), when there is the temperature change of about 30° C. in the objective lens with the NA more than 0.70 formed of the plastic material, there is a possibility that the recording and/or reproducing of the information is hindered. Further, when the objective lens whose NA is large and whose diameter is small, is composed of 2 positive lenses, because the working distance tends to be small, there is a problem that a possibility in which the objective lens is brought into contact with the optical information recording medium by the warping of the optical information recording medium, is large.
In view of the above described problems of the conventional technology, an object of the present invention is to provide an objective lens which is low cost and light weight in the same manner as the conventional plastic single lens, although it is a high performance objective lens, corresponding to the increase of the numerical aperture (NA) of the objective lens.
Further, an object is to provide an objective lens for recording and/or reproducing of the optical information recording medium in which the applicable temperature range is large, even when it is a high NA objective lens composed of 2 positive lenses formed of plastic material.
Further, an object is to provide an objective lens for recording and/or reproducing of the optical information recording medium in which the diameter is small and the working distance is large, even when it is a high NA objective lens composed of 2 positive lenses.
Further, an object is to provide a coupling lens by which the axial chromatic aberration generated in the objective lens due to the mode hop phenomenon of the laser light source or the high frequency superimposition can be corrected, and which is structured at low cast.
Further, an object is to provide a light converging optical system and an optical pick-up apparatus by which the variation of the spherical aberration generated in each optical surface due to the change of the oscillation wavelength of the laser light source, temperature•humidity change, or the error of the thickness of the transparent substrate of the optical information recording medium, can be effectively corrected by a simple structure.
Further, an object is to provide a light converging optical system and an optical pick-up apparatus by which the axial chromatic aberration generated in the objective lens due to the mode hop phenomenon of the laser light source or the high frequency superimposition can be effectively corrected.
Further, an object is to provide a light converging optical system and an optical pick-up apparatus which is provided with a short wave laser light source and an objective lens with high numerical aperture, and by which the information can be recorded or reproduced for the optical information recording medium having a plurality of recording layers with the transparent substrate among them.
In this connection, as the high NA objective lens whose NA is larger than 0.7, a lens in which, when the refracting power is distributed to 4 surfaces by structuring by 2 positive lenses, and the radius of curvature of each surface is increased, the error sensitivity at the time of the metal mold processing or lens molding is moderated, is proposed. However, when the objective lens in which the NA is large in this manner and the diameter is small, is structured by 2 positive lenses, because the working distance tends to be reduced, there is a problem that the possibility in which the objective lens is in contact with the optical information recording medium due to the warping of the optical information recording medium, is large.
Further, because the plastic lens has large change of the refractive index and the shape due to the temperature as compared to the glass lens, the performance deterioration thereby easily becomes problem. This performance deterioration, that is, because the increase of the spherical aberration is larger as the NA is increased (generally, it is increased in proportion to 4-th power of the NA), in the objective lens formed of the plastic lens with more than NA of 0.7, when the temperature change is about 30° C., there is a possibility that the trouble occurs in the recording and/or reproducing of the information.
Further, another problem which is actualized in the reduction of the wavelength of the laser light source and the increase of the numerical aperture of the objective lens, is the variation of the spherical aberration generated in the objective lens due to the minute variation of the oscillation wavelength of the light source. The semiconductor laser used as the light source in the optical pick-up apparatus has the dispersion of ±10 nm among individual elements. When the semiconductor laser having the oscillation wavelength dislocated from the reference wavelength is used as the light source, because the spherical aberration generated in the objective lens is increased as the numerical aperture is increased, the semiconductor laser having the oscillation wavelength dislocated from the reference wavelength can not be used, and it is necessary to select the semiconductor laser to be used as the light source.
When the high density next generation optical disk as described above comes into the practical use, for the recording and reproducing apparatus•optical pick-up apparatus for such the high density optical disk, the interchangeability is required so that the recording•reproducing is possible also for even the conventional optical disk such as the DVD.
Further, presently, although the diffractive optical element is used as the high performance aberration correction element in the optical pick-up apparatus, in such the diffractive optical element, in order to prevent the decrease of the diffraction efficiency, it is important that the diffractive structure is formed in the shape as close as possible to the design value. However, when the diffractive optical element is used for the optical pick-up apparatus using the objective lens of the short wavelength light source such as the blue violet semiconductor laser in which the practical use is presumed in the near future as described above, or the high numerical aperture, because the diffracting power necessary for the correction of chromatic aberration is increased, the period of the diffractive structure, for example, the interval of the blaze of the blaze structure is about several times of the using wavelength, that is, about several μm. Presently, in the diamond super precision cutting technology (SPDT) generally used as the production method of the metal mold of the diffractive optical element, because the shape of the tip of the bite is transferred onto the step difference of the blaze structure, the phase unconformity portion is surely generated. Therefore, in the case of the diffractive structure in which the period is so small degree as several times of the using wavelength, because the influence of the phase unconformity portion is largely appears, there is a problem that the sufficient diffraction efficiency can not be obtained.
As the widely known method to form the diffractive structure having the small period of about several μm, the electronic beam drawing method exists, and the flow of the formation of the diffractive structure by this method is as follows. Initially, the photo resist is coated on a board, and next, while the electronic beam exposure apparatus is scanning on the photo resist, the electronic beam exposure amount distribution corresponding to the shape distribution of the diffractive structure is given. Next, the photo resist is removed through the etching process and the plating process, and the convex and concave pattern of the diffractive structure is formed on the board. As the diffractive element in which the minute blaze structure is formed on the plane board by such the electronic beam drawing system, a diffractive lens according to Optics Japan 99', 23a A2 (1999) is known.
However, in the diffractive optical element used for the optical pick-up apparatus, because the diffracting action by the diffractive structure and the refracting action in the refractive surface are adequately combined and the aberration correction is conducted, there is a problem that the above diffractive lens in which the diffractive structure is formed on the plane board having no refracting power can not be used for the optical pick-up apparatus as the aberration correction element.
The object of the present invention is to provide a coupling lens, light converging optical system, optical pick-up apparatus, recording apparatus and reproducing apparatus, by which the axial chromatic aberration generated in the objective lens due to the mode hop phenomenon of the laser light source is effectively corrected.
Further, the object of the present invention is to provide a diffractive optical element which is an optical element having the diffractive structure used for the optical pick-up apparatus, and which has the shape in which the diffractive structure can be formed by the electronic beam drawing system, and the optical pick-up apparatus provided with such the diffractive optical element.